Glycolic Acid

Glycolic acid is the smallest alpha hydroxy acid, with just two carbon atoms. Size is the whole story with this ingredient.

Why molecule size matters

Being the smallest AHA, glycolic acid penetrates faster and further than lactic, mandelic or malic acid at the same concentration. That makes it the most effective of the group and also the most likely to irritate. The two are the same property, not a trade-off you can formulate around.

It was originally derived from sugar cane, though most cosmetic glycolic acid is now synthesised.

How it works

Glycolic acid interferes with the bonds between corneocytes in the outermost layer, allowing them to shed more evenly rather than accumulating unevenly. That is the direct exfoliating effect.

Longer-term studies have also reported changes in the dermis with sustained use, including effects on glycosaminoglycan and collagen content. That is a slower and less direct effect than the surface exfoliation, and it depends heavily on concentration and duration.

Concentration and pH together

Percentage on its own tells you very little. Glycolic acid needs to be partly un-ionised to be active, which depends on pH. A 10% product at pH 4 behaves quite differently from 10% at pH 3, and pH is rarely disclosed.

Format matters just as much. In a rinse-off cleanser, contact time is under a minute. In a leave-on pad or serum, it is hours. Comparing percentages across formats is not meaningful.

The sunburn warning

AHA products above certain strengths carry a mandatory sun warning in several markets, typically specifying protection during use and for a period afterwards. This is not marketing caution. Exfoliating the outermost layer reduces one of the skin's own defences against UV, and the effect persists after you stop.

Daily broad-spectrum SPF is a requirement of using glycolic acid, not an optional extra.

Who should be careful

Dry, sensitive or reactive skin, rosacea, and anyone using retinoids concurrently. Deeper skin tones carry an additional consideration: over-exfoliation can trigger post-inflammatory hyperpigmentation, meaning an aggressive approach to uneven tone can worsen it.